This invention is related to light emitting diode, in particular indium gallium nitride light emitting diode.
In the III-V compound semiconductor family, the nitrides have the highest direct bandgap. The light emitting wavelength lies in the ultra-violet to yellow color range. The material is suitable for fabricating short-wavelength, high efficiency light emitting devices. Although a great deal of effort have been devoted to commercialize the product, the goal has not been achieved due to the following reasons:
1. No suitable substrate material to match the crystalline structure of the nitride material. PA1 2. Difficulty in growing indium-gallium-nitride (InGaN), especially that with high indium content. PA1 3. Difficulty of growing high p-type GaN materials; PA1 4. Difficulty in forming good electrode contact to the material.
Not until the end of 1993 did the Nichia Chemical Industries Ltd. in Japan announce the successful fabrication of a blue light emitting diode using gallium nitride as the basic material. Afterwards, a high intensity green light emitting diode was also developed. At present, many research organizations have invested a large amount of resources to pursue such development, but only a few two and three companies have achieved any success. The foregoing difficulties are the bottlenecks.
As shown in FIG. 1, the Nichia Chemical Industries disclosed in a paper that the InGaN light emitting diode uses a sapphire substrate 18, a GaN nucleation layer 17A, an n-type GaN buffer layer 16, an n-type GaN cladding layer 15, a InGaN quantum well light emitting layer 14, a p-type aluminum-gallium-nitride cladding layer 13, a p-type GaN contact layer, a nickel-gold (Ni/Au) light transmitting electrode 11A, a p-type Ni/Au electrode 10. Due to the fact that the sapphire is an insulator, the light emitting die is Selectively etched to the n-type GaN cladding layer 16 and forms an n-type titanium/aluminum electrode 19.
The p-type GaN contact layer 12, after thermal annealing, usually has a carrier concentration of less than 1.times.10.sup.18 cm.sup.-3. The lowest resistivity is no lower than 1 ohm-cm Such poor conductivity cannot effectively distribute the current over the entire semiconductor die and causes a current crowding phenomenon which lowers the light emitting efficiency.
The prior art shown in FIG. 1 uses a very thin Ni/Au layers 11A. The thickness is only few hundred Angstroms as a current spreading layer to effectively spread the current over the entire die. However, such a Ni/Au layer 11A has a transmittance of less than 50%. Thus, the major portion of the light emitted from the light emitting diode is absorbed by the current spreading layer to lower the light emitting efficiency.